Signal and noise control system



1943{ R HOLLINGSWORTH 2,337,196

SIGNAL AND NOISE CONTROL SYSTEM Filed March 2'7, 1942 2 Sheets-Sheet lZ6 70 2 0 5; JPEA R FAMPL.

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2 Sheets-Sheet 2 R L. HOLLINGSWORTH SIGNAL AND NOISE-CONTROL SYSTEMFiled March 27, 1942 Dec. 21, 1943.

' INVENTOR 15.5 1 /044 wgsn aam.

\ ATTORNEY Patented Dec. 21, 1943 SIGNAL AND NOISE CONTROL SYSTEM R LeeHollingsworth, Riverhead, N. Y., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application March 27, 1942, SerialNo. 436,421

8 Claims.

The present invention relates to automatic gain control systems, andmore particularly to an improved signal and noise control systemsuitable for use both in telegraph and telephone systems.

In known types of threshold automatic gain control (AGC) systems, nowcommonly used for telegraph receiving operation, if the threshold ismanually set high when the incoming signals are rather weak and theseincoming signals become strong in a short period of time, there is atendency for the AGC action to split the telegraph characters at thestart as they pass through the receiver, if the fastest range of AGC isused, as determined by the values of the time constant circuit. Thefastest range is seldom used for relatively strong signals in theseknown systems, except for facsimile keying speeds, Where the slowestkeyed impulses occur at a rate faster than the time delay action of theAGC. The fastest range is used for comparatively weak signals Where verydeep rapid fading is encountered, in order to prevent short drop-outs ofthe keyed signal to th line from the output of the receiver that takeplace within the time duration of a keyed impulse under theseconditions.

The difficulty mentioned above, experienced with known AGC systems, maybe better understood by reference to the graphical representationsillustrated in Figs. 1a, 1b and 10.

Fig. 10. represents the rectified output of a telegraph receiver, asmight be observed in an oscilloscope while receiving weak signals withfast AGC, in known systems. The horizontal dotted line represents thepoint at which a succeeding tone keyer will be manually set to operate.

Fig. 1b represents the rectified output of the same telegraph receiverin known systems, as might be observed in an oscilloscope for a strongsignal using fast AGC. It should be noted that the transient eiTect atthe top and beginning of each character dips down below the originalpoint of keyer operation. This transient effect, here represented byWiggly lines, is caused by the output of the receiver feeding back tothe input through a relatively fast time constant to control the overallgain of the receiver.

Fig. 1c illustrates the effect of this undesirable transient of Fig. 1bon the tone keyer output of the receiver. It should be noted, in Fig.1c, that the tone characters for the dot and the dash are split at thoseportions corresponding to the points at which the transient of Fig. 1bdips down below the keyer operation point. As mentioned before, thissplitting of the characters is highly undesirable because of thedistortion that it creates in the signal output of the telegraphreceiver.

One of th objects of the present invention is to overcome the foregoingdifficulty of split telegraph characters caused by using the fastestrange of AGC in telegraph receivers.

Another object of the present invention is to produce a faster actingAGO system to minimize or eliminate completely the efiect of strong peaknoise, such as might be caused by ignition, sharp static, or suddenbursts of signal.

Still another object of the invention is to enable the transmission ofsloped telegraph characters which can be converted at the receiver tosquare wave characters for proper recording.

By the use of a telegraph receiving system, in accordance with oneembodiment of the invention, employing transmitted character of slopedform, the apparatus at the transmitter can be of simple design and thesystem as a whole can utilize a reduced frequency spectrum in thetransmission medium between the transmitter and the receiver, comparedto a system where the transmitted characters are of square wave form.

In accordance with one embodiment of the invention useful for telegraphreception, the control to the AGC circuit increases automatically as thesignal increases above a predetermined level, and this control at thesame time automatically sets the input voltage to the tone keyer to ahigher voltage to prevent splits from showing in the keyed signal atslow keying speeds if very fast AGC is used. In another embodiment ofthe invention, particularly applicable to peak noise re duction ofspeech or modulated wave reception, the time constant circuit of thereceiver is shunted or by-passed on the charge cycle, thus reducing, inpart, strong short duration static impulses. This last embodiment makesno use of a keyer circuit.

A more detailed description of the invention follows in conjunction withthe drawings, Where- 1n:

Figs. 1a, 1b and 1c graphically illustrate observed signals in telegraphreceivers of the prior art;

Fig. 2-illustrates a telegraph receiver in accordance with oneembodiment of the present invention.

Figs. 3a., 3b, 3c and 3d graphically illustrate the transmission ofsloped characters and the conversion to square wave tone impulses at thereceiver in accordance with one embodiment of the invention and Fig. 4illustrates a speech or modulated wave receiver in accordance withanother embodiment of the invention.

In Figs. 2 and 4, the same parts are represented by the same referencenumerals.

Figs. 111, lb, 10 have been described above in connection with adescription of the difficulties experienced with known types oftelegraph receivers, and for this reason need not be further referred toin greater detail.

Referring to Fig. 2, there is shown a telegraph receiver having adirectional multi-dipole antenna l coupled to a radio frequency vacuumtube amplifier 2 the output from which is coupled to multi-stage vacuumtube apparatus 3 representing a mixer or frequency converter and one ormore subsequent intermediate frequency amplification stages. Oscillator22 is the vacuum tube heterodyne oscillator which beats with the signalsimpressed upon the mixer stage in apparatus 3 for producing intermediatefrequency energy. The intermediate frequency output from apparatus 3passes through an inductance coil L1 which is coupled to a pair of coilsL2 and L3. Coils L1, L2 and L3 are individually tuned by means ofvariable condensers (as shown) to the desired intermediate frequency.Connected to the high potential end of coil L2 is a rectifier tube 4whose rectified output passes to ground through serially connectedresistors 5 and B. The high potential end of coil L3 is connected torectifier tube 1 whose output also passes to ground primarily throughresistance 6. It will thus be seen that when rectifier tube 1 isconductive, resistor 5 will be predominantly by-passed or almost shortcircuited by the current passing through this rectifier. Resistors illand I! which are preferably of like value and condenser l2 form a timeconstant circuit for supplying AGO action over lead AGC to the radiofrequency amplifier 2 and to one or more of the intermediate frequencyamplifier stages of apparatus 3. Suitably connected to a point onresistor 6 by means of a tap and through a battery it are a tone keyertube !5 and a threshold tube l4 whose control electrodes are connectedelectrically in parallel relation. The output or the anode circuit ofthreshold tube M is connected through polarizing battery It and througha resistor ii to ground, a point on which resistor is connected by meansof lead 24 to one terminal of resistor H of the time constant circuit.The output or anode circuit of tone keyer I5 is conn cted to groundthrough resistor [3. Apparatus 23 is a balanced modulator systemcomprising, in effect, a relay constituted by a pair of vacuum tubes i9and 20 whose corresponding input and output electrodes are connected inpush-pull relation. A source of audio frequency tone 9 is connected toboth halves of the input coil coupled between the grids of the tubes [9and 29. An audio frequency transformer 2i constitutes the output circuitof these two tubes for supplying keyed tone signals to the line TL, forutilization by a suitable recorder (not shown). The balanced modulator23 is well known and forms no part of the present invention per se.

When the tone keyer tube I5 is conductive and passes current, the biason the tubes 59 and 20 caused by the IR. drop through resistor I8 issuflicient to prevent the tubes is and 2%] from passing current, inwhich condition the tone from source b will not be passed through thebalanced modulator to the line TL. However, when the tone keyer tube I5is non-conductive, there will be no current passing through resistor I8, as a result of which the negative bias on tubes [9 and 28 is removedto an extent sufficient to render these tubes conductive to allow tonefrom source 8 to pass through transformer 2! to the line TL forrecording purposes.

Threshold tubes I l and are normally conductive in the absence ofsignals, as a result of which current will flow through resistors l! and[8. From What has been said above, the flow of current through resistoris will prevent current from being passed through the line TL. The flowof current through resistance ill will supply a negative voltage to lead2% and thence to the time constant circuit by Way of resistor H, as aresult of which negative bias is supplied to the AGC lead during theabsence of incoming wave signals.

Rectifier tube i is so connected to the circuit through coil L3 thatthis tube does not pass current until the intensity of the incomingsignals is above a predetermined level, whereas rectifier tube d is soconnected to the circuit through coil L2 that it passes signals at amuch lower level of intensity. To obtain this diiference in operationor" the rectifier tubes i and i at the different signal levels, the coilL2 may be coupled more closely to coil L1 than coil L3, oralternatively, the same result may be obtained by having both coils L2and L3 coupled similarly to coil L1 and placing a negative bias ofpredetermined magnitude on the grid of rectifier l by means of battery8. Obviously, the operation of the two rectifier tubes 4 and i can beobtained by a combination of a difference in coupling of the coils L2and L3 to coil L1 and by a suitable selection of the bias on the grid oftube '5.

The operation of the system of Fig. 2 will now be given. Let us assumethat the rectifier tube is so biased or coupled to the system that itwill operate when the intensity of the incoming signal is at apredetermined value above the signal intensity necessary to producerectification in tube 1. By Way of example, and not by way oflimitation, rectifier tube 1 may commence rectifying when the signalintensity is ten times that required to produce rectifying action intube 4-. Upon receiving a weak signal, not sufficient to causerectification of tube 1, tube 4 will rectify the signal to produce aflow of current through resistors 5 and 6. The voltage drop across 6will, through the time constant circuit l0, H and I2, control the gainof apparatus 2 and 3 by way of lead AGC. A portion of the voltage dropthrough resistor 6 will be applied to the control grid of threshold tube14 and reduce the conductivity of this tube to thereby reduce the fiowof current through resistor ll. The reduction in current flow throughresistor [1 will have a corresponding effect on the AGC lead by virtueof connection 24. During the time of this weak received signal impulse,if the voltage originally selected by the tapping point of lead 24 onresistor I1 is less than the signal voltage supplied across resistance 6and available at point A, the gain of the receiver will increase duringthis received signal impulse. On the other hand, if the original biasvoltage selected by the position of the tapping point on the resistor Eland supplied to the AGC circuit by lead 24 is greater than the signalvoltage across resistor 6 and available at A, the gain of the receiverwill be decreased during said received signal impulse. If the weaksignal which reduces the current in threshold tube I4 is not immediatelysufiicient to cut ofi the flow of ourrent through resistor H, thereduction in the current through resistor l7 and the increase of thegain in the receiver by virtue of the AGC action will increase the gainof the signal to an extent which will completely out oit current throughtube It. In the telegraph system in which it is contemplated using thisreceiver, particularly in a diversity receiving arrangement, thepresence of a weak signal will be sufficient to immediately cut off theflow of current through tube 14. Since tubes l4 and i5 are preferablycontrolled in parallel, the action which cuts off the flow of spacecurrent in tube 54 will simultaneously cut off current through tonekeyer tube l5 and thus permit tone from source 9 to pass out over theline TL. When the signal intensity of the received signal increasessharply beyond the critical level at which tube l commences to rectify,for reasons heretofore mentioned, the fiow of current through tube llwill produce a low impedance path in shunt to resistor 5, thus in effectby-passing resistor 5. Thus, the flow of current through the rectifier Ewill cause a considerable increase in the current flow through resistorS, as a result of which a considerably large voltage will be availableat point A for use in the AGC lead to more positively control the gainof the receiver in such direction as to reduce the gain. Thus, it shouldbe evident that any sharp increase of signal strength, such as may becaused by static crash or a strong signal burst, will immediately reducethe gain of the receiver and increase the negative voltage applied tothe control grids of tubes i l and i5, thus preventing splits ormutilations in the characters of the telegraph signals sent out over theline TL.

By using a system of the type shown in Fig. 2, it is possible to reducethe band width required in the frequency spectrum of the medium betweenthe transmitter and the receiver. band width is preferably reduced bythe transmission of characters with sloping sides instead of standard oraccepted square wave character transmission, as at present in generalpractice. This may be more readily understood by referring to the graphsof Figs. 3a to 3d, inclusive. In Fig. 30. there are shown signalcharacters comprising a dot and dash of the generally accepted form oftransmitted signals, in accordance with known systems. According toknown practice, a central oi'hce will transmit signal characters ofsquare wave form which because of the line faci ities become distortedand of sloping character before the signals arrive at the transmittingor radiating point. It has been customary, heretofore, at thistransmitting or radiating point to convert the sloping characters tosquare wave characters before transmission over the ether. By means ofthe present invention, we can directly transmit the sloping charactersover the ether without the need for converting them to square wavecharacters at the transmitting point. Since the transmission of squarewave form at the transmitter requires a wider frequency band than thetransmission of sloping characters, I am able to reduce the band widthof the frequency spectrum between the transmitter and the receiverrequired for a certain speed of transmission. This is done bytransmitting over the ether signal characters having sloping sides, inthe manner shown in Fig. 3b, and by converting these characters ofsloping form, when received, to square wave form as originallytransmitted from the central office.

Fig. 3c graphically shows how this is done in the telegraph receivingsystem of Fig 2. The gain of the receiver is adjusted by means of atapping point on resistor H to a relatively low value. As the signalbegins to pass through the receiver by way of rectifier 4, resistance 5and 8 to ground, threshold tube it begins its process of being cut oil.During the process of this tube cutting off, the gain of the receiver(due to a reduction in the flow of current through resistance ll) willgreatly increase to a point where rectifier 1 suddenly comes intoaction, increasing the current through resistance 6 and thereby furtherreducing the gain of the receiver but at the same time considerablyincreasing the negative bias applied to the control grids of tubes i4and I 5. In effect, rectifier tube 1 provides a snap action whichdecreases the gain of the receiver suddenly in order to cut off sharplythe how of current to tone keyer E5 to thereby produce square wave toneoutput characters from the balanced modulator 23. By suitable selectionof the values of anode battery l6 for tube I l and the anode battery 25for tube l5 and the grid bias battery lit for tube M, the diiferentialpoint of operation between tubes M and can be chosen to make thenonconductive point of operation of tube l5 coincide with the operationof the rectifier tube "i. The point of operation of the tone keyer i5 isrepresented by point B on curve 3C. From what has gone before, it willbe apparent that the fiow of current through tube 14 is not immediatelycut off and that current flows through tube i l from point C on graph 30to point B, in a continuously decreasing amount, and that when point Bis reached, both tubes Hi and I5 cease conducting altogether. The tonesignal characters in line TL are represented by graph the durations ofwhich correspond to the fiat top portions of curve 36.

The automatic volume control circuit of the present invention operatesas explained in my two patents, Numbers 2,104,324 and 2,243,423. In theabsence of carrier wave energy, threshold tube I4 is conductive,supplying a negative automatic gain control voltage supply that ispassed through resistance ii to control the gain of the receiver. Withcarrier current present causing rectifier tube t to become conductive,furnishing an automatic gain control supply voltage across resistance 6,threshold tube M- is wholly or partially cut oif, in turn wholly orpartially eliminating the negative voltage drop across resistance ll.Therefore, there is a presence of negative voltage supply at all timesavailable to the automatic gain control circuit to properly control thereceiver in such a manner that the gain is never allowed to rise to apoint where noise would become excessive in the output circuits.Furthermore, if the automatic gain control voltage manually tapped fromacross resistance ll is exceeded during incoming wave signals by thevoltage drop across resistance 6, the gain of the receiver will befurther reduced. If the signal voltage developed across resistance 5decreases as it normally would du ing a condition of fadthe gain of thereceiver will rise to a value corresponding to or within the noise levelsignal values permitting same. If the voltage developed acrossresistance 6 happens to be exactly equal to the voltage developed acrossresistance it, the gain of the receiver does not change; the automaticgain control supply is simply fed automatically through resistances l8and H during signal or no signal conditions.

It will be appreciated that there is an equalization efi'ect produced bythe continual interchanging of the automatic gain control supply fromresistance 5 and from across resistance H, as a result of voltagevariation across resistance 6, which partially cancels the modulationcomponents feeding into the automatic gain control circuit. Forinstance, let us assume that a carrier is being received withoutmodulation. There is little or no bias at all supplied from resistancell due to reduced conductivity or complete cutoff of threshold tube E4.The automatic gain control bias is then fed through resistance llJ.However, when modulation peaks rise to a height to be of consequence, byfeeding through resistance it into condenser l2 and which tend todemodulate or cause distortion to the incoming Waves through theautomatic gain control circuit, threshold tube l4 becomes conductive tosupply a negative voltage through resistance H to replace the electronsin the automatic gain control circuit taken away momentarily from acrossresistance 6 during modulation peaks.

Fig. 4 illustrates a receiver of the present invention which isprimarily for use in receiving speech or modulated Wave signals. Theelements of Fig. 4 which bear the same reference numerals as those ofFig. 2 are substantially the same and operate in similar fashion. Sincethe receiver of Fig. 4 is intended only for telephonic or modulated wavesignals, it lacks the tone keyer of Fig. 2 and includes the audiotranslating circuit here represented by audio frequency transformer 25,the primary winding of which is connected across the resistor 5. Itshould be noted that the control grid of tube it of Fig. 4 obtains partof its bias through a cathode resistor 2'! which is preferably employedin this particular receiver rather than the use of the bias battery I3shown in Fig. 2. It should also be noted that the lower terminal of coilL3 is coupled to the time constant circuit through a resistor 28 whichis relatively low compared to the value of resistor H. The time constantcircuit also has a resistor Hi which in this case is also of a low valuecompared to the resistor H,

As for the operation of Fig. 4, normally this receiver operates as astandard modulated wave receiver with the additional noise reductioncharacteristics provided by the use of rectifier F tube '7 and itsassociated elements. Let us assume that the receiver is designed to passWith equal gain the signals between one and fifty decibels, and thatrectifier tube 1 becomes conductive when the signal intensity approachesthe upper limit of the modulated Wave, let us say at around forty-eightdecibels. The reception of a strong static crash or strong signal aboveforty-eight decibels would thus cause rectifier l to become conductiveand cause an increased flow of current through resistor E. An increasedcurrent will fiow through low resistor 28 directly to the AGC lead,causing the gain of the receiver to instantaneously decrease sharply forthe duration of the stat c crash. Because of the relatively low value ofresistor l8 compared to resistor l l, a general increase in the flow ofcurrent in resistor 6 will occur, by virtue of which the modulated waveappearing in transformer 26 will be slightly increased during theduration of the static crash. Explaining further, the current throughelements consisting of rectifier i, inductance L3, resistance 28,resistance l0 and resistance 6 to ground potential, encounters lessresistance than through resistance 5. Therefore, since resistance 5 issuitably placed in parallel With a resistance of less value, the totalplate load circuit of rectifier t is lessened, causing the rectifiedcurrent to increase to ground through resistance 6. This slight increasein the intensity of the modulated wave during the excessively strongincoming signal will prevent a drop out or complete cut oif of thesignal appearing in the translator coupled to transformer 25, during theinterval of the excessively strong signal. Putting it in other Words,the system of Fig. 2 prevents a hole from being knocked out of theincoming modulated wave, as available for utilization, during the staticcrash, and this hole knock out is prevented by by-passing the currentfrom rectifier tube 1 through low resistors 28 and IE! to therebyincrease the current through resistor 6. By suitable choice of theresistors 28 and 10', we can obtain equalization between the voltageadded to the AGC circuit and the voltage added across resistor 6 tothereby give the desired effect of substantial reduction or of apparentreduction in the modulation output.

In the operation of Fig. l, if it is desired to Wholly or partiallyremove a strong static impulse, the anode circuit of rectifier tube 7may be connected through coil L3 directly to ground. Upon becomingconductive, due to an excessive static impulse or excessive modulationlevels, the output of rectifier l during this period of time becomes avery low impedance circuit, thus bypassing the excessive noise energy orexcessive modulation energy to ground around the audio output system;but on its return to its cathode, it meets and opposes the flow ofcurrent through resistors 5 and 6 produced by rectifier 4, thus creatingopposing voltages which wholly or partially equalize themselves, thusproducing an effect of partial compression of the noise or modulationcomponents. Another Way of explaining this cancellation effect is thatthe rectified current passing through resistors 5 and 6 is suddenlyconnected through the low resistance path of rectifier 1 to ground, as aresult of increased signal or noise excitation in inductance L3 frominductance L4. During extreme signal peaks the electrons collected atthe top end of resistance 5 suddenly pass through rectifier 1 creating apartial deficiency of electrons for the moment at the top of resistance5. Thus, as the cathode return current of rectifier l replaces thisdeficiency of electrons, an effect equal to partial cancellation exists,reducing the noise or excessive modulation components appreciablythrough resistance 6.

The term ground used in the specification and claims denotes any pointor surface of fixed alternating current potential or of zero radiofrequency potential.

What is claimed is:

1. In a signal receiving system, a signal transmission tube, a rectifiertube including a resistive connection in its space current path acrosswhich is developed a direct current voltage varying in magnitude withsignal intensity variation, an automatic gain control system comprisinga time constant circuit including a condenser-resistor combinationconnected between a gain control electrode of the transmission tube and.a point on the resistive connection which becomes more negative withsignal intensity increase, and another rectifier tube in shunt to aportion of said resistive connection located between said time constantcircuit and said first rectifier tube,

both of said rectifier tubes being individually coupled to the output ofsaid transmission tube and so constructed and arranged as to becomeconductive at different values of incoming signal voltage, said secondrectifier tube controlling the rate of charge of the condenser in saidtime constant circuit and the rapidity of response of said gain controlsystem in response only to large increases in signal voltage, to therebyreduce the effective resistance of said portion and thereby decrease theeffective time constant for strong signals.

2. In a receiving system, an amplifier adapted to amplify the incomingwaves, means for rectifying the products of said amplifier, a timeconstant circuit including a resistance and a con denser in circuit withsaid means for impressing a potential obtained from said rectified wavesupon said amplifier to control its gain, a direct current impedancelocated between said time constant circuit and said means through whichthe rectified currents pass to thereby limit the rate of charge of saidtime constant circuit, and an electron discharge device in shunt to saiddirect current impedance and coupled to the output of said amplifier,said device being only responsive to an increase in the intensity of theincoming waves above a predetermined value for providing a path of lowimpedance across said direct current impedance for strong signals, saidpredetermined value being appreciably above the value at which saidmeans begins rectifying the products of said amplifier, whereby saidcondenser is more rapidly charged to increase the speed of control oversaid amplifier.

3. In a signal receiving system, a signal transmission tube, a rectifiertube including a resistive connection in its space current path acrosswhich is developed a direct current voltage varying in magnitude withsignal intensity variation, -:a

automatic gain control connection between a gain control electrode ofthe transmission tube and a point on the resistive connection whichbecomes more negative with signal intensity increase, said automaticgain control connection including a, time constant circuit having aseries resistor and a condenser connected between a point on saidresistor and a surface of fixed alternating current potential, andanother rectifier tube in shunt to a portion of said resistiveconnection, both of said rectifier tub-es being individually coupled tothe output of said transmission tube and so constructed and arranged asto become conductive at difierent values of incoming signal voltage,said second rectifier tube being responsive only to sudden largeincreases in signal voltage, an electron discharge device having itsinput electrode coupled to a point on said resistive connection outsideof said shunt portion and its output electrode connected through aresistor to said surface of fixed alternating current potential, and aconnection from a point on said last resistor to a point on the seriesresistor of said time constant circuit, and means for biasing the inputelectrode of said device such that said device is conductlve in theabsence of incoming signals and in this condition supplies a negativevoltage to said gain control connection.

4. In a radio telegraph receiving system, a signal transmission tubehaving a parallel tuned output circuit, a second parallel tuned circuitcoupled to said first tuned circuit, a rectifier tube having anelectrode coupled to the high potential terminal of said second tunedcircuit and another electrode coupled to the low potential terminal ofthe second tuned circuit through a pair of series resistors, a thirdparallel tuned circuit coupled to said first tuned circuit, said thirdtuned circuit being in shunt to one of said pair of series resistorsover a path including another rectifier tube, said three parallel tunedcircuits being tuned to the same frequency, an automatic gain controlcircuit including a time constant element between a gain controlelectrode of the transmission and a point on the other resistor of saidpair, said second and third tuned circuits being so coupled to saidfirst tuned circuit that said rectifier tube is responsive to a lowerintensity of incoming signal than said other rectifier tube, a t eiieyer tube having its input electrode connec d to a point on the otherresistor oi said pai and its output electrode coupled a relay system,the bias on the input electrode of said tone keyer being such that thekeyer is conductive in the absence oi signals and is nonconductive inthe presence oi signals, to thereby control the operation of said relayaccordingly.

In a radio telegraph receiving system, a signal transmission tube havinga parallel tuned output circuit, a second parallel tuned circuit coupledto saidf'rst tuned circuit, a rectifier tube having an electrode coupledto the high potential terminal oi said second tuned circuit and anotherelectrode coupled to the low potential terminal or the second tunedcircuit through a pair of eries resistors, a third parallel tunedcircuit coupled to said first tuned circuit, said third tuned circuitbeing in shunt to one or said pair of series resistors over a pathincluding another rectifier tube, said three parallel tuned circuitsbeing tuned to the same i'requency, an automatic gain control circuitincluding a time constant element between a gain control electrode ofthe transmisslon tuoe'and a point on the other resistor of said pair,said second and third tuned circuits being so coupled to said firsttuned circuit that said first rectifier tube responsive to a lowerintensity or incoming signal than said other rectifier tube, a tonekeyer tube having its input electrode connected to a point on the otherresistor of said pair and its output electrode coupled to a relaysystem, a threshold tube having its input electrode in electricallyparallel relation to the input electrode of said tone lseyer and itsoutput electrode connected through a resistor to ground, and aconnection from a point on said last resistor to said time constantelement, whereby the fiow of current through said last resistor suppliesa negative voltage to said automatic gain control circuit, the bias onthe input electrodes of said threshold and tone lieyer tubes being suchthat they are conductive in the absence of signals and becomenon-conduct1ve during the reception of signals.

In a modulated wave receiving system, a signal transmission tube havinga parallel tuned output circuit, a second parallel tuned circuit coupledto said first tuned circuit, a rectifier tube having an electrodecoupled to the high potential terminal of said second tuned circuit andanother electrode coupled to the low potential terminal 01 the secondtune-d circuit through a pair of series resistors, a third paralleltuned circuit coupled to said first tuned circuit, said third tunedcircuit being in shunt to one of said pair of series resistors over apath including another rectifier tube, said three parallel tunedcircuits being tuned to the same frequency, an automatic gain controlcircuit including a time constant element between a gain controlelectrode of the transmission tube and a point on the other resistor ofsaid pair, said second and third tuned circuits being so coupled to saidfirst tuned circuit that said first rectifier tube is responsive to alower intensity of incoming signal than said other rectifier tube, avacuum tube having its input electrode connected to a point intermediatethe ends of the other resistor of said pair of series resistors and itsoutput electrode connected to ground through a resistor, a connectionfrom a point on said last resistor to said time constant element, thebias on the input electrode of said last vacuum tube being such that thetube is conductive in the absence of signals during which time itsupplies a negative bias to said automatic gain control circuit, and anaudio frequency transformer having its primary winding connected acrossthe other resistor of said pair of series resistors and its secondarywinding coupled to a utilization circuit.

'7. In a multi-stage system, an intermediate stage vacuum tube waveamplifier, a transformer having an input winding and two outputwindings, connections between the output of said amplifier and saidinput winding, a first rectifier tube connected to one of said outputwindings, an output resistance for said first rectifier with aconnection to a succeeding input transformer, a

second rectifier connected across the other output transformer windingand adjusted to become conductive after the first rectifier becomesconductive, both of said rectifiers being connected in electricallyparallel relation to said output resistance, whereby the application ofa strong Wave to said amplifier increases the IR drop across saidresistance, thus producing an expanding characteristic for said strongamplified wave.

8. In a multi-stage system, an intermediate stage vacuum tube waveamplifier, a transformer having an input winding and two outputwindings, connections between the output of said amplifier and saidinput winding, a first rectifier tube connected to one of said outputwindings, an output resistance for said first rectifier with aconnection to a succeeding input transformer, a second rectifierconnected across the other output transformer winding and adjusted tobecome conductive after the first rectifier becomes conductive, both ofsaid rectifiers being connected reversely to said output resistance sothat the opposing voltages partially cancel the stronger waves, wherebya limiting or compression action is produced as the waves pass throughsaid Wave amplifier.

R. LEE I-IOLLINGSWORTH.

